Speaker
Description
We present a 10-Gbps 4-channel VCSEL driver with an on-chip charge pump which automatically increases the power supply voltage to ensure enough voltage headroom to the VCSEL diode operating in radiation and low-temperature environment. The charge pump efficiency is above 75%. An automatic control circuit is implemented to adjust the power supply voltage to the VCSEL. The rest of the design is based on an earlier prototype VLAD14, and will be fabricated with a 65 nm CMOS process in mid -May.
Summary
Optical links working with lpGBT will need an optical driver operating up to 10.24 Gbps per fiber. The optical transceiver VTRx+, the optical transceiver that is being developed in the Versatile Link Plus common project, has power supplies at 1.2 V and 2.5 V. The 2.5 V power supply is only sufficient to drive the VCSEL with a forward voltage below 2.0 V. VCSELs from several vendors have a forward voltage that is very close to even above 2.0 V. Tests indicate that when operating in high radiation and in the low-temperature environment (such as the silicon trackers at LHC), the forward voltage of the VCSELs approaches 2.5 V. Without changing the powering scheme in the LHC experiments, we have designed a 4-channel VCSEL driver that has an on-chip charge pump increases the power supply voltage to provide enough voltage headroom for the driver. The design is based on an earlier prototype VLAD14 reported in TWEPP2017 and meant to provide a driver ASIC for VTRx+.
The on-chip charge pump adopts a compact voltage doubler structure in which there are two boosting capacitors driven by two complementary clock signals in 1.2 V voltage domain. The boosting capacitors are charged by a 2.5V power supply and provides current alternatively to the output capacitor through switches. The charge pump generates ripple at the output when loaded. The ripple amplitude depends on a switching frequency and size of boosting and output capacitors. We design the oscillator running at 800 MHz in order to limit the on-chip capacitor size. Each VCSEL driving channel has its own charge pump. The boosting capacitors and the output decoupling capacitor is 60 pF for each. We combine both Metal-insulator-Metal (MIM) and (Metal-Oxide-Metal) MOM capacitor to maximize the capacitance density. With the load of 18 mA current at 3.3V, the ripple of the charge pump is less than 10 mV and the efficiency is above 75%.
We design a control circuit adjusting the charge pump voltage while VCSEL forward voltage is changing with radiation and temperature. The DC voltage at the anode of the VCSEL, depends on the VCSEL threshold voltage, is sensed and used to adjust the charge pump output voltage. The amplifier in the loopback is implemented with 1.2 V core devices to avoid the high voltage IO devices, which are not suitable for applications in radiation.
The rest of the design is based on an earlier prototype VLAD14 which has been measured up to 14 Gbps. The high-speed signal is AC coupled to the current amplifier which is now powered by the charge pump with a voltage varies from 2.5 to 3.3 V. Devices in the current amplifier are carefully designed to avoid the overdriven problem.
The whole design is completed and will be submitted in May this year. We expect to get the test results before the conference meeting.
